Two American companies which provided encrypted email services – one to the NSA fugitive Edward Snowden – have abruptly shut down the service, apparently following US government pressure to let it read users' messages.

Lavabit, which is believed to have been used by Snowden and which claimed to have 350,000 customers, closed after apparently rejecting a US government court order to cooperate in surveillance on its customers by allowing some form of access to the encrypted messages on its servers.

Its founder Ladar Levison wrote on the company's website: "I have been forced to make a difficult decision: to become complicit in crimes against the American people or walk away from nearly ten years of hard work by shutting down Lavabit."

Snowden – whose whereabouts are still unclear – told Guardian columnist Glenn Greenwald that Lavabit's decision to close rather than provide access to user content was "inspiring" and asked why internet giants such as Google and Facebook "aren't fighting for our interests the same way small businesses are."

Silent Circle, another American company which had offered encrypted email – where messages stored on its server would be unreadable – also announced on Thursday that it was ending its "Silent Mail" service, "to prevent spying".

After the Lavabit shutdown, "We see the writing on the wall, and we have decided that it is best for us to shut down Silent Mail," the company said in a statement. "We have not received subpoenas, warrants, security letters, or anything else by any government, and this is why we are acting now… It is always better to be safe than sorry, and with your safety we decided that in this case the worst decision is no decision."

It said that it would still offer its Silent Phone and Silent Text offerings, which provide encrypted voice and text messaging: "We don't have the encrypted data and we don't collect metadata about your conversations," it said – an allusion to the collection by the NSA of metadata about phone calls made in the US from phone carriers including Verizon.

Mikko Hypponen, chief research officer at online security company F-Secure, told the Guardian: "From what Lavabit said, I assume they were asked to put in a back door to give access, or weaken their encryption, and they didn't want to do that." He said that encrypted email services were useful to "all sorts of people – whistleblowers, people working under totalitarian governments, but also people who want to send threatening emails or plan bad things. It's like cash. Drug lords use that. So do you and I."Privacy advocates called Lavabit's decision unprecedented. "I am unaware of any situation in which a service provider chose to shut down rather than comply with a court order they felt violated the constitution," said Kurt Opsahl, a lawyer with the Electronic Frontier Foundation.

Meanwhile, the whereabouts of Snowden, whose revelations about the NSA's work have proved so explosive, remain unclear.

Snowden's lawyer Anatoly Kucherena said the National Security Agency leaker had registered his place of residence with the authorities, as all foreigners must do, but did not divulge the location out of concern for his safety. A migration service official said on Friday, however, that Snowden had not submitted documents to register in the capital.

"We don't have these records. I don't have any such information. He's not in Moscow," said Olga Kirillova, head of the Moscow branch of the Federal Migration Service, news agencies reported.

Snowden's father Lon has submitted the necessary documents for a visa to visit Russia and plans to come in August, his lawyer said. Kucherena told journalists earlier this week that